Hypothalamic regulation of growth hormone secretion during food deprivation in the rat

Suppressed pulsatile GH secretion in food-deprived rats has been hypothesized to be due to an increase in hypothalamic somatostatin secretion. We investigated this hypothesis and the role of GHRH in regulating GH secretion during food deprivation using two different models. In experiment one, rats were food deprived for 72h during which time they received a saline infusion (n = 5). At the same time rats were normal fed for 72h during which time they received a somatostatin infusion (5 micrograms/h, n = 7). After the 72h infusion period, all rats received two iv injections of GHRH (1 microgram/rat) at 2h intervals. GH concentrations in food-deprived rats rose from approximately 10 ng/ml to 400-800 ng/ml in response to both GHRH injections. This increase was significantly greater (p < 0.01) than the GH response (100-400 ng/ml) observed in somatostatin-infused animals. The significantly higher GH response observed in food-deprived rats as compared to somatostatin-infused, normal-fed rats suggests that somatostatin concentrations may decrease during food deprivation. In experiment two, rats were infused for 5h with either saline (n = 6) or GHRH (10 micrograms/h, n = 9) at the end of a 72h fast. GH concentrations did not change in saline-infused animals. In contrast, GH concentrations significantly increased (p < 0.01) upon initiation of the continuous GHRH infusion. Yet, this release of GH was pulsatile in nature. Pulsatile GH secretion in the presence of a constant GHRH infusion suggests that pulsatile somatostatin release from the hypothalamus is maintained during food deprivation. These studies suggest that during food deprivation in the rat 1) absolute concentrations of somatostatin decrease, but its pattern of secretion remains pulsatile, and 2) decreased GHRH release may be responsible for the absence of spontaneous GH pulses.